Numerous types of speedometers are known in the art for measuring the speed of marine craft relative to the water in which the craft is travelling. One type of conventional marine speedometer utilizes a pitot tube that is disposed in the water to produce pressure within the pitot tube in response to the movement of the craft. Tubing is connected between the pitot tube directly to a mechanical indicator. The pressure created in the pitot tube is used to mechanically drive a needle in the indicator to indicate the speed of the craft. Craft speed is proportional to the square root of the pressure in the pitot tube. Since the pressure is not linearly proportional to the speed of the craft, the mechanical indicator typically includes a nonlinear scale to indicate the speed of the craft relative to water.
This type of marine craft speedometer is often inaccurate. The length of tubing between the pitot tube and the indicator varies with the size of the marine craft and with the location of the indicator in the craft. The tubing tends to dissipate and reduce the pressure response created by the pitot tube with the amount of reduction in pressure being proportional to the length of the tubing. Thus, varying the length of the tubing introduces inaccuracies in speed indication.
As a marine craft travels through waves or choppy water, the pressure on the pitot tube often varies quite rapidly so that the instantaneous pressure on the pitot tube is not proportional to the speed of the marine craft. Under these conditions, the indicator needle of conventional mechanical speedometers bounces, and accurate indications of speed are lost.
A conventional speedometer using a nonlinear scale is also hard to read, and often an operator is confused or mislead by such nonlinear indications of speed. For example, if the needle points to a position one half the distance between 0 and 10 mph, the operator may interpret this reading as 5 mph when in fact the speed is indicated to be approximately 7.07 mph. Many waterways and harbors have a speed limit of 5 mph. An operator of a marine craft with a conventional nonlinear mechanical speedometer as described above may misread such speedometer and unintentionally violate the speed limit.
Another type of conventional marine craft speedometer uses propellers or paddle wheels to measure the speed of the craft relative to the water. However, this type of speedometer is unduly expensive, is generally unreliable and suffers from problems similar to those discussed above.
Thus, a need has arisen for a marine craft speedometer in which the length of the pitot tube plus the tubing connected to the pitot tube is of a predetermined length which is not varied according to the size of the marine craft or the location of the indicator relative to the pitot tube. In this construction, the pressure in the pitot tube will correspond to the speed of the craft in a known manner. Particularly, a need has arisen for an electronic speedometer in which the pressure in a pitot tube is sensed and an electrical voltage is generated in response thereto. This electrical voltage may be transmitted to a volt meter for indicating the speed of the craft. Regardless of the location of the volt meter (the indicator), the length of tubing between the pitot tube and the sensing device is known and is constant. Also, in an electronic speedometer, the voltage output of the sensor device or transducer can be buffered with reactance or integrated over a period of time to produce a smooth indication of the speed of the craft. In this manner, the bouncing needle problem associated with mechanical speedometers is eliminated. Furthermore, a need has arisen for an electronic speedometer producing a voltage that is linearly proportional to the speed of the craft so that a digital indicator (volt meter) or a linearly scaled indicator may be used to indicate speed instead of conventional speed indicators with nonlinear scales.